A sprawling wildfire in northern Colorado grew larger than the nearby city of Fort Collins on Monday, racing swiftly across a crackling dry landscape, spitting flames as far as 300 feet into the air and forcing thousands out of their homes.

The Red Cross, Humane Society and other aid groups mobilized to help evacuees while at least 400 firefighters, aided by air tankers and helicopters from as far away as Canada battled the fire about 15 miles west of Fort Collins.

"The hope for containment today, I will tell you, is tenuous," incident commander Bill Hahnenberg said.

The fire nearly doubled in size overnight to 36,930 acres, or more than 57 square miles, Larimer County Sheriff's Office Executive Officer Nick Christensen said Monday. It had been estimated at 20,000 acres Sunday night.

The rapid growth made the fire zone larger than Fort Collins, which is 47 square miles.

Photos: Northern Colorado fire rages on 24 photos

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – CNN iReporter Bryan Maltais of Fort Collins, Colorado, took this picture of the Colorado fire from the cliffs above Horsetooth Reservoir in Larimer County. He started documenting the situation on June 10. "Many people I know have been brought to tears for the people who have lost their homes, and for the destruction of so much forest," he said.

Hide Caption

1 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Jessie Couillard watches the High Park Fire a few miles from her house in Glacier View Meadows, northwest of Fort Collins, on Monday, June 18. The lightning-ignited wildfire that started June 9 has destroyed nearly 200 homes.

Hide Caption

2 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – The High Park Fire rages west of Fort Collins on Monday. The blaze has ravaged more than 58,700 acres but is about 50% contained, authorities say.

Hide Caption

3 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – A truck offers a message for firefighters west of Fort Collins on Monday. More than 1,700 personnel are battling the wildfire.

Hide Caption

4 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Smoke from the High Park Fire rises above the Colorado National Guard Readiness Center near Fort Collins on Sunday, June 17.

Hide Caption

5 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – A view of the High Park and its large plume of smoke from Fort Collins on Wednesday, June 13.

Hide Caption

6 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Resident Bill Janz reunites with his dog Abby on Wednesday. Abby had been at Janz's home when the fire started. She jumped into a fire truck as it approached the house, escaping the fire.

Hide Caption

7 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Pat Wright hugs daughter Stephanie Lauger after finding out Wednesday the High Park Fire hasn't destroyed her house. She will be able to return home three days after evacuating.

Hide Caption

8 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – A large smoke plume rises from the High Park Fire west of Fort Collins on Wednesday.

Hide Caption

9 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – People watch smoke from the High Park Fire billow. The fire has consumed more than 50,000 acres and jumped Highway 14, prompting evacuations.

Hide Caption

10 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Steve Segin of the U.S. Forest Service shows a map of the size and location of the northern Colorado blaze Tuesday, June 12, in Laporte, Colorado.

Hide Caption

11 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – Residents watch the approaching fire Tuesday in the Roosevelt National Forest west of Fort Collins.

Hide Caption

12 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – iReporter Victor W. Schendel, a Fort Collins photographer, has been watching the fast-moving wildfire grow and took this photo Tuesday as firefighters attempted to control the blaze.

Northern Colorado fire rages on – The High Park Fire -- about 15 miles west of Fort Collins -- doubled in size overnight to 36,930 acres, or more than 57 square miles, authorities said Monday.

Hide Caption

19 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – An orange-tinted sky looms over a small barn near Laporte on Sunday.

Hide Caption

20 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – The fire is behaving erratically, fueled by strong winds, low humidity, high temperatures and dry brush, an official says.

Hide Caption

21 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – A firefighting helicopter surveys the High Park Fire for hot spots Sunday near Laporte as the wind-driven blaze spreads.

Hide Caption

22 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – A satellite image from NASA shows a plume of smoke from the High Park Fire about 20 miles northwest of Fort Collins on June 9.

Hide Caption

23 of 24

Photos: Northern Colorado fire rages on24 photos

Northern Colorado fire rages on – The sprawling wildfire lights up the night sky Saturday in a photo from CNN iReporter Randy S. Macht taken in Louisville, Colorado, south of the blaze.

Hide Caption

24 of 24

EXPAND GALLERY

While Fort Collins was not immediately threatened by the fire, a smoky pall hung over the city of 143,000, said Stephanie Ashley, a spokeswoman for the Larimer Humane Society.

"It's pretty much a haze covering the town, and you can definitely smell it," said Ashley, whose shelter was housing more than 170 animals dropped off by evacuees.

The fire was visible from the Colorado State University campus in Fort Collins, where students and staff were told to move activities inside if possible. The university's Foothills campus was serving as command center for the firefighting efforts.